New research has disproved the idea that the extinction of the dinosaurs allowed mammals the chance to take over the earth. Modern mammal groups only diversified sometime after the mass extinction. But if dinosaurs weren’t holding them back, what was?

While the mighty dinosaurs walked the earth, the ancestors of modern mammals were scurrying through the undergrowth beneath their feet, biding their time. Sixty-five million years ago, their opportunity came.

During a massive extinction event, the majority of life on earth including the entire dinosaur line (with the exception of the birds) went extinct. And like ambitious young graduates whose boss got the sack, the mammals took their ecological place.

They rapidly diversified into a variety of different forms, eventually giving rise to the four thousand plus species that exist today. Right?

Wrong.

This is the picture that has been painted by scientists, textbooks and popular culture for decades. But in the light of new evidence, it just doesn’t measure up.

Olaf Bininda-Emonds, Andy Purvis and a team of international scientists reconstructed the tree of life that unites almost all living species of mammals. Using both fossils and genetic evidence, they worked out how modern species split apart from common ancestors over the last several million years.

Their study turned up many surprises. For a start, they found that the seeds of modern mammal dynasties were planted much earlier than expected.

Almost twice the expected number of mammal groups were around to see the K/T boundary ­– the point in time where the dinosaurs and their peers went extinct. While the dinosaurs were still stomping about, the early mammals were busy exploiting smaller ecological niches.

But after the K/T boundary, the researchers found no sign of the expected rapid expansion of mammal lineages. As it turns out, mammals – or our forefathers at the very least – were not impatient go-getters ready to spring into action when opportunity knocked. In evolutionary terms, they were slackers.

The fuse that led up to the explosion of modern mammal groups was clearly longer than we thought. And Bininda-Emonds’s work suggests a reason for that too.

As it happens, some mammal groups did diversify quickly after the dinosaurs’ coup de grace, giving rise to species like Andrewsarchus (right), a ferocious hooved predator. But these animals too eventually went extinct. Today, they have no living descendants.

These temporarily dominant groups, like the dinosaurs before them, may have kept our own ancestors in the evolutionary shade. As Purvis explains, “For the first 10 or 15 million years after the dinosaurs were wiped out, present day mammals kept a very low profile, while these other types of mammals were running the show.”

Perhaps they adapted to a still-changing climate only to be wiped out mere geological moments later. The forefathers of today’s mammals could have fared better because they played it slow and steady, and waited until conditions settled down before diversifying.

One Response

A new paper has just come out in Nature that contradicts this study. Here’s what New Scientist has to say:

“According to the fossil record, our ancestors didn’t split into modern groups of placental and marsupial mammals until after the dinosaurs bit the dust at the end of the Cretaceous, 65 million years ago. So say John Wible of the Carnegie Museum of Natural History in Pittsburgh, Pennsylvania, and colleagues, who have compared late Cretaceous fossils with modern placental groups.

Biologists consider the first split among modern mammalian groups as marking the origin of placentals, a key point in evolutionary history. Wible’s comparison found that none of the Cretaceous fossils belong to modern groups (Nature, vol 447, p 1003). He concludes that placental groups split away shortly after the Cretaceous, and “explosively evolved” to fill niches opened by the death of the dinosaurs.

That bolsters the traditional view of palaeontologists, but flies in the face of molecular studies of genetic divergence of living species, which put the origin of placentals 80 to 140 million years ago (New Scientist, 28 March, p 18). “We’re in total discord with the molecular dates,” Wible says. He thinks genetic clocks fail to account for the post-Cretaceous burst of mammalian evolution.

Are palaeontologists missing fossils, or do bursts of evolutionary diversification throw off molecular clocks? You have to take both sides seriously, says Rich Cifelli of the Oklahoma Museum of Natural History in Norman.”

You can find more detailed write-ups of this paper from Brian at Laelaps

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